A rail union has said it will consider strike action over the threat of “potentially lethal” legionella bacteria found on Thameslink trains.

Trace amounts of legionella were found in seven toilets on four trains.

Thameslink said the toilets had been drained and bleached but the Rail, Maritime and Transport (RMT) union said this was “half-hearted and inadequate”.

The train operator said there was “no recorded case of anyone, ever, contracting legionella from a train”.

The union said there were seven toilets on four trains that had “actionable traces” of the bacteria, which can cause Legionnaires’ disease.

Left untreated Legionnaires’ disease – a lung infection caused by legionella bacteria – can be fatal.

Initial symptoms are flulike and include a high fever and muscle pain. The disease is treated with intravenous antibiotics.

Legionella bacteria are commonly found in sources of water, such as rivers and lakes, and sometimes find their way into manmade water supply systems. The bacteria are also linked to a milder flulike illness called Pontiac fever.

The union has called for an urgent meeting of Thameslink’s Joint Safety Committee.

RMT general secretary Mick Lynch said the union had been “raising concerns for weeks now”.

“The latest cavalier approach from the company is pitifully inadequate and is an outright gamble with passenger and staff health.

“We have now declared a dispute. Be in no doubt, if we don’t get serious action we will ballot our members and do whatever is required to end this reckless approach to a potentially lethal situation on these increasingly busy trains.”

Thameslink said no other parts of its trains were affected by legionella, including its air-conditioning systems which do not use water, and the fact that its toilets were “gravity fed” further lowered any risk of spreading the bacteria.

The company added that all its trains were being tested but there were none in service known to have traces of legionella.

Train services director Rob Mullen said: “A very low level of legionella was found to be present during testing in a small number of our Thameslink Class 700 train toilets.

“While it is extremely unlikely this would cause any harm to passengers or colleagues, the toilets affected were immediately locked out of use.

“The trains were taken out of service and these toilets have now been drained, bleached and had their tanks completely refilled.”